Apple is happy to take 0% of offsite payments if you don’t link to them though, so clearly apple is fine with an app store fee of 0, ie for spotify.

The only difference is if the app error says “You can’t pay in-app” or if it says “You can’t pay in app, you can pay on our site”

Restricting apps from informing users still seems like obvious harm to users, like if a retail store made a rule that “the manual that comes with your product can’t contain your homepage because you have an online shop that might have better prices than us”

If it were the cost of running the store, truly, it would charge based on app downloads or such. Not based on if users click a link to amazon.com in the kindle app and then buy 2 books or 3.